I. Field of the Invention
The present invention relates to a process of extracting chromium from chromium ores, more specifically to a process for treating chromium ores with oxygen.
Chromium derivatives have a wide field of application, especially in the manufacture of pigments, tanning of hides, as surface coatings, in oxidation reactions in the chemical industry and in metallurgical industries.
II. Description of the Prior Art
Chromium occurs with a valence of three in its naturally occurring ores. The commonest of these ores is chromite, a natural oxide of iron and chromium, which in some cases contains oxides of magnesium, aluminum and silicon. Chromium is generally extracted from its ores by oxidation in the form of chromate, in which state its valence is six. The product of this reaction is then treated to extract the chromate. The chromate in solution is acidified with sulfuric acid to form the bichromate, an intermediate product commonly used in the preparation of the various chromium derivatives.
In the present state of the art, the commonly used process for oxidation of chromium ores is known as oxidizing alkaline roasting.
Many variations have been proposed to the roasting operation which is the essential step in the oxidizing alkaline roasting process. Ordinarily, the oxidizing reaction is carried on in a rotary furnace in the presence of sodium carbonate and calcium carbonate, or dolomite, the function of which is to neutralize the acid oxides, which are always present in the ore, and thus to impede their going into solution during the extraction step which follows. To achieve high oxidation yields compatible with an acceptable treatment cycle, the mass of reagents must be brought to a temperature above 1000.degree.C. At this temperature, the sodium chromate and carbonate have passed their melting points. The result is the appearance of a molten phase forming a sticky, pasty mass in the furnace, which by its very nature causes many operating difficulties. Despite the remedies proposed, it has not been possible hitherto to keep this pasty mass from sticking to the walls of the furnace. In addition, the oxidation reaction is slowed down due to difficulty encountered by the oxygen in penetrating the paste. In order to mitigate these adverse effects an inert solid mass is usually added to the reaction mixture, which acts as a diluent for the molten phase. The residues from the aqueous extraction step after roasting are often recycled for this purpose. While this recycling has a favorable effect, it entails a burdensome increase in the aggregate bulk of equipment and lowers the overall heat efficiency of the furnace.
In another proposed modification, to overcome some of the disadvantages noted above, agglomeration by briquetting the raw materials before roasting has been suggested.
All these modified roasting processes intrinsically involve many disadvantages, such as high operating temperature, the need for recycling, and the resultant wasted space in the rotary furnace. Consequently, it would be especially desirable to have a simpler means of oxidative smelting of chromium ores.
It is well known, to those skilled in the art, that chromium ores may be oxidized in a bath of molten Cr.sub.2 O.sub.3 with sodium peroxide, but this process is not commercially feasible due to its high cost, unlike the process of this invention.